This invention relates to electric power supply apparatus of the kind that converts an alternating input voltage into a unipolarity output voltage, and it relates more particularly to such apparatus wherein the average magnitude of the output voltage is maintained equal to or higher than a predetermined minimum level regardless of variations in the amplitude of the input voltage and/or in the direct current load that is supplied by the apparatus.
There are many practical applications for a voltage reducing power transformer having a primary winding connected to an alternating current (a-c) electric power line or main of relatively high voltage and constant frequency (e.g., 60 Hertz) and having multiple secondary windings that are respectively connected to several different electrical load circuits of lower voltage. Where a load circuit requires direct current (d-c), a rectifying type of power supply apparatus needs to be provided between such a load and the associated secondary winding of the transformer. Such apparatus typically comprises a plurality of unidirectional electrical valves arranged in a full-wave bridge configuration having a pair of a-c input terminals connected to the associated transformer secondary winding and a pair of d-c output terminals connected to the load circuit.
The average magnitude of the unipolarity output voltage of such power supply apparatus depends on the amplitude and waveform of the input voltage. The input amplitude is subject to variations for various reasons. The amplitude of the line voltage applied to the primary winding of the power transformer may vary over a relatively wide range. Even if the line voltage were constant, the amplitude of the alternating voltage across the associated secondary winding will vary due to transformer regulation when there are variations in the load current that is flowing either in that winding or in another secondary winding of the same transformer. In order to minimize its size and cost for a given power rating, a transformer is conventionally designed to have relatively high reactance; consequently, the voltage drop in the internal impedance of the transformer changes appreciably as the magnitude of load current increases or decreases. There is another factor that will affect the average magnitude of output voltage in those applications where another secondary winding of the transformer is connected to a load circuit that includes a phase-controlled rectifier bridge. This factor is notches in the input voltage waveform during the recurrent intervals when the other winding is short circuited by the commutation action of the phase-controlled rectifier bridge.
Where the average magnitude of voltage applied to a d-c load circuit must be prevented from falling below a predetermined minimum, the power supply apparatus needs to include suitable means for increasing the output voltage during low input voltage conditions. This can be done in a variety of different ways. One way is to build the power transformer with less reactance, but this solution would increase the transformer size and cost. Another way involves increasing the voltage rating of the transformer secondary winding, using controllable valves (e.g., thyristors or silicon controlled rectifiers) for the unidirectional valves in the full-wave rectifier bridge of the power supply apparatus, and providing control means for "phase controlling" the bridge so that only part of the rectified input voltage waveform is normally applied to the load circuit when its amplitude is high but all is applied during abnormally low input voltage conditions. The disadvantages of this solution are the added costs of the thyristors and their controls, and the larger size and higher cost of the additional filter capacitors that would be required to attenuate the higher ripple content of the rectified input voltage under normal conditions.
A more practical way to increase the output voltage is to add a voltage step-up autotransformer and two pairs of suitably controlled inverse-parallel thyristors, as is disclosed, for example, in U.S. Pat. No. 3,263,157. In such prior art voltage regulators the autotransformer typically withstands the input voltage continuously, and therefore its power and thermal ratings would be relatively high and its size, weight and cost would be undesirably large. In addition, an undesirable amount of power would be dissipated and wasted as heat.